The present invention relates generally to computer networks and specifically to failover or redundancy in network equipment.
The OSI model developed by the International Organization for Standards (ISO) serves a guideline for developing standards for data communication. Different pieces of network equipment are governed by these standards and allows for the interconnection of various network equipment.
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The seven layers (L1 to L7) are briefly summarized as follows:                a. Application (Layer 7)—This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. This layer provides application services for file transfers, e-mail, and other network software services.        b. Presentation (Layer 6)—This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept.        c. Session (Layer 5)—This layer establishes, manages, and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.        d. Transport (Layer 4)—This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.        e. Network (Layer 3)—This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.        f. Data Link (Layer 2)—At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.        g. Physical (Layer 1)—This layer conveys the bit stream—electrical impulse, light or radio signal—through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.        
Network data switching equipment, such as that equipment used for switching or routing of information packets between network devices, handle data at the lower layers of the OSI model, while application level programs handle data at the higher OSI layers. It is desirable for network switching equipment to remain in operational condition for continuous periods of time.
A common administrative activity is the installation of new software or software modules. In most installations, it is desirable to avoid or at least minimize the impact of bringing down the system for such tasks. Whereas halting a higher level application for a software upgrade activity typically affects only the user or users of the application, halting operation of network switching equipment can impact a larger community of users, indeed possibly the entire enterprise. Software updates to applications are relatively easy to do because only a relatively limited number of users are usually affected. By comparison, the downtime that may result from performing software updates to network switching equipment could affect an entire enterprise.